The present invention relates to the field of high-temperature vacuum furnaces, and provides a method and apparatus for rapid and safe cooling of such a furnace.
Vacuum furnaces are well known in the art, and are used for heat treating of articles at very high temperatures, of the order of 2000.degree. F. or higher. Vacuum furnaces have been used for many different kinds of heat treatment of metallic objects, such as bright hardening, annealing, sintering, and brazing, and other purposes. At the high temperatures required for such processes, the components being treated are very prone to chemical reaction and oxidation, necessitating a non-reactive furnace atmosphere.
Vacuum furnaces can be configured for continuous operation, or as batch-type furnaces. In the former case, the material to be heat treated is passed through the furnace on a conveyor means; in the latter, the material is placed in the furnace and heated, and then the furnace is cooled together with its contents. The present invention is primarily concerned with batch-type vacuum furnaces.
A perennial problem in the operation of vacuum furnaces has been in the cooling step. Because the temperature in the furnace is so high, a long time is required to bring the temperature down. When one attempts to accelerate the cooling process by injecting a cold liquid into the furnace, the sudden contact with the cold liquid can cause thermal shock, and warp or otherwise damage the contents of the furnace. A slight mistake in the amount of cooling liquid introduced at one time can render the furnace unusable.
Of course, a slower cooling process is, in general, safer for the furnace. But reducing the rate of cooling also impairs the overall productivity of the furnace, because a longer cooling process means that the furnace is out of service for longer periods of time.
It has been known, in the prior art, to introduce a cooling gas, or even a liquefied gas, into a vacuum furnace to promote cooling. It is necessary that such a cooling gas be relatively inert, because of the likelihood of chemical reaction at the high temperatures in the furnace. Thus, for example, U.S. Pat. No. 3,860,222 shows a cooling system for vacuum furnaces, wherein argon is used as a cooling gas. Similarly, U.S. Pat. No. 3,522,357 discloses a vacuum furnace wherein argon is introduced into the furnace prior to quenching by water or oil. U.S. Pat. Nos. 4,395,832 and 3,565,410 teach the use of nitrogen as the inert gas to be injected into the furnace.
Another process for cooling a furnace and its contents is described in the paper entitled "The Cryogenic Rapid Cooling Process", by Dr. Werner Buecker, prepared for the 8th ASM Heat Treating Conference, Detroit, Sept. 18, 1984, the disclosure of which is incorporated by reference herein. This paper provides a method of cooling which can be used for both continuous and batch-type furnaces, the method including the spraying of liquid nitrogen directly on the object to be cooled. The above-described process is also disclosed in U.S. Pat. No. 4,515,645. The disclosures of the patents cited above are incorporated by reference herein.
The present invention comprises a process and apparatus which significantly reduce the time required to cool a vacuum furnace. The apparatus includes automatic means for regulating the cooling process. Although a cryogenic liquid is employed, in the present invention, to cool the cooling gas, at no time is any cryogenic liquid allowed to enter the furnace itself. The process disclosed herein is also more economical than prior art methods, because it uses a less expensive gas for cooling, to the extent possible.